One of the first applications of synthetic polymer synthesis in coatings technology was the alkyd resin. It was successful in chemically combining oil or oil-derived fatty acids into a polyester polymer structure, thereby enhancing the mechanical properties of these coating vehicles over those of the oils alone.
A typical alkyd resin is a polyester made by reacting an unsaturated fatty acid, a dibasic carboxylic acid, and a polyol. Usually, the dibasic acid is phthalic anhydride or isophthalic acid, and the polyol is trifunctional. The reaction of the dibasic carboxylic acid with the polyol produces the backbone of the polyester, thereby building its molecular weight. In turn, the fatty acid groups attach to this backbone and, in the presence of oxygen and a metal catalyst, serve to cure the resin into the final crosslinked film.
Resins are typically classified by the amount of fatty acid that they contain, i.e. their "oil length". The oil length of an alkyd is defined as the weight of any fatty acid present plus the weight of the polyol needed to completely esterify this fatty acid (minus the weight of evolved water from the esterification), expressed as a percentage of the total weight of the finished resin. Oil length (OL) is divided into three classes: short-, medium-, and long-oil. A medium-oil alkyd contains between 45 to 55% oil, with short-oil alkyds containing less and long-oil alkyds containing more oil. Long-oil and medium-oil lengths are commonly used in brush-applied solvent-based decorative coatings, while short-oil alkyds are found in automotive and general industrial stoving compositions.
Traditionally, solvent-based alkyds have obtained their good film properties by a combination of high resin molecular weights coupled with drying from pendant, unsaturated fatty acids present in the alkyd. While contributing to desirable film properties, the high molecular weight of the resin also causes the resin to have an unsuitably high viscosity . . . thus necessitating low-solids formulations for coating applications. Due to environmental concerns and increasing governmental regulations on the volatile organic content (VOC) of coatings, coating producers are attempting to find alternatives to these conventional low-solids level, solvent-based formulations.
One approach to address the VOC regulations is the development of water-based formulations. While the use of water as a solvent is attractive, it has proven difficult to obtain the properties and performance of a solvent-based coating via a water-based system. These developmental problems are especially pronounced in the areas of high gloss and corrosion resistance.
The other primary approach to the VOC regulations has been to reduce the amount of volatile organic solvent necessary for a formulation by reducing the average molecular weight of the alkyd resin. A reduction in molecular weight, while reducing resin viscosity and allowing an increased solids level coating to be feasible, also has proven deleterious to the properties of the final coating. This so-called high-solids approach involves developing resins that are soluble in organic solvents at levels as high as 90 to 95 weight percent solids. These high-solids levels must be attained while maintaining a low viscosity to allow for formulation and application of the coating.
Therefore, it is the object of this invention to have an alkyd composition that addresses environmental concerns by significantly reducing the viscosity of a resin without decreasing the average molecular weight of the resin. In addition, organic solvents are frequently expensive and any means of reducing their presence could lead to lower costs for coatings.